Bacteria belonging to the genus Bifidobacterium are major bacteria in the human intestinal bacterial flora and are known to have beneficial effects on human health, such as regulation of intestinal function, for example, improvement of constipation and diarrhea, suppression of an increase in serum cholesterol, and immunostimulation. For this, a number of commercial products containing the bacteria belonging to the genus Bifidobacterium are available in the forms of various fermented foods and drinks, probiotic preparations, and the like. Particularly, fermented milk foods and drinks have excellent palatability; therefore, they are suitable for continuous ingestion of the bacteria belonging to the genus Bifidobacterium. 
The bacteria belonging to the genus Bifidobacterium are obligate anaerobes and susceptible to oxygen, low pH, and high acidity. Thus, there are many difficulties in handling the bacteria belonging to the genus Bifidobacterium in fermented milk foods and drinks in terms of proliferation during production, viability during storage, and the like. In order to obtain the physiological effect of the bacteria belonging to the genus Bifidobacterium, it is considered necessary to deliver the bacteria alive to the intestine as many as possible. Particularly, increasing the viability of the bacteria in foods and drinks, namely the rate of arrival of ingested bacteria at the intestine, is regarded as an important factor.
In order to solve the above problems, an attempt has been made to improve the viability in fermented milk foods and drinks by improving the production method and adding various viability-improving agents such as N-acetyl glucosamine, pantothenic acid, peptides, and lactulose. However, such a viability-improving agent for the bacteria belonging to the genus Bifidobacterium cannot be easily added because not only it increases the production cost but also it causes problems such as reduced palatability. Further, a method that completely blocking the bacteria belonging to the genus Bifidobacterium from contacting oxygen by filling a fermented product containing the bacteria into a container composed of an oxygen-impermeable packaging material immediately after the product is produced is also studied. However, no perfect oxygen-impermeable container has yet been available. Further, there is not a lot of flexibility in shaping such a container, and waste disposal is complicated since the container is made by using composite materials. Furthermore, the container itself is expensive, etc. As mentioned above, there are many limitations in using the container.
Accordingly, a fundamental solution for improving the viability of the bacteria belonging to the genus Bifidobacterium in fermented foods and drinks is to produce bacteria belonging to the genus Bifidobacterium having high viability even under aerobic conditions and under conditions of low pH and high acidity. Examples of such a bacterial strain include Bifidobacterium breve YIT 10001 (FERM BP-8205) (Patent Document 1), Bifidobacterium breve SBR 3212 (FERM P-11915) (Patent Document 2), and Bifidobacterium bifidum YIT 4002 (FERM BP-1038) (Patent Document 3).
However, there has been a problem such that these strains with improved viability are expected to exhibit their effects of improved viability only under conditions in which they are produced. That is, in the production of a mutant strain of the bacteria belonging to the genus Bifidobacterium, a method of subculturing and storing bacteria belonging to the genus Bifidobacterium in an environment which is harsh for the bacteria to grow and obtaining a surviving strain is normally practiced. While a certain level of viability-improving effect can be expected in an environment under which the mutant strain has been produced, no viability-improving effect can be expected in other environments. Accordingly, the bacteria belonging to the genus Bifidobacterium obtained by a conventional method cannot be applied to foods and drinks which are distributed under a condition different from that under which the mutant strain is produced. Accordingly, utility of the conventional method has been extremely limited.
Also, although the cause is unknown, it is known that even a bacterial strain with improved viability obtained by subculturing and storing under conditions with deteriorated environmental factors (for example, a pH in the acidic region) does not exhibit its viability-improvement effect when used under mild conditions at a pH in the neutral region. This has been also one of the reasons for failing to obtain a highly-versatile bacterial strain which is applicable to various foods and drinks.